Preheat your oven to 350 degrees F. Spray a 9-inch x 13-inch baking dish with non-stick cooking spray. Set aside.
Prepare the cake mix according to the package directions. Pour the cake batter into the prepared dish. Bake according to the package time for a 9x13 baking dish.
Remove the cake from the coven and let it cool completely on a wire rack, about 2-3 hours.
Using a small wooden spoon, dowel, or fork, poke holes in the top of the cake in rows at 1-inch intervals. (Be sure to clean your poking tool as you go so the holes don't get bigger due to stuck-on cake.)
Pour the lime jello into one mixing bowl and the cherry jello into a second mixing bowl. Bring the water to a boil and then pour 1 cup of water into each bowl. Whisk until the gelatin has melted.
Using a spouted measuring cup, pour the lime jello down one column of holes and then cherry down the column next to it. Repeat alternating columns until all the holes are filled.
Refrigerate the cake for 3-4 hours.
Spread the Cool Whip all over the top of the cake and then add your favorite Christmas sprinkles over the Cool Whip. Chill until ready to serve.
Notes
If you prefer to use a homemade cake recipe, this Vanilla Cake Mix recipe is great. Use a 9x13 baking dish as directed and then bake for 25-33 minutes, or until a toothpick inserted in the center comes out clean.